The present invention relates in general to intake manifolds for internal combustion engines. More in particular, it relates to manifolds having means for preventing reverse flow of fuel and air mixture in the runners of the manifold and for increasing the amount of fuel and air mixture delivered to an engine.
A carbureted internal combustion engine employs an intake manifold to distribute a fuel and an air mixture produced by a carburetor to the cylinders of the engine.
An intake manifold typically has a plenum chamber below the carburetor to receive a mixture of fuel and air from the carburetor. From the plenum the mixture is directed to the cylinders through ducts called runners. The runners exit from the manifold at inlet ports to the engine. These ports lead to the cylinders through inlet valves. When each cylinder has an associated runner not shared with another cylinder, the manifold may be called an independent runner manifold.
A fuel and air mixture is drawn into each cylinder of an engine by a vacuum created there by downward piston movement during the intake stroke of the cylinder. Inlet and exhaust valves into each cylinder provide for the admission of the fuel-air mixture into the cylinder and the exhaustion of products of combustion from the cylinder. In modern internal combustion engines, both the inlet and exhaust valves are open at the same time during a portion of a cycle. Some of this overlap occurs while the cylinder's piston is ascending. At this time the inlet valve is just opening and the exhaust valve is finally closing. A reason for the overlap is to give a fuel-air charge in a runner of a manifold the opportunity to see a large inlet valve opening when the piston begins to descend in the cylinder and produce a vacuum. However, during the overlap the pressure within a cylinder is greater than that in the manifold and this differential is compounded by the ascending piston. Consequently, exhaust products pass through the inlet valves from the cylinders they serve and into the runners of the manifold. These exhaust products tend to produce reverse fuel and air mixture flow towards the carburetor by "pushing" the mixture.
An engine acts as a pump when it produces the vacuum that constitutes the driving force acting on a fuel-air charge to force the charge into the cylinders. This pumping requires power and the more power required to get a given mass of charge into the cylinders the lower the engine's efficiency. Power loss attends reverse mixture flow. To the extent that the flow of exhaust gases into a manifold requires the admission from the manifold of these exhaust gases back into the cylinder, work is required and efficiency is lost. The engine simply has to pump more to get both the fuel-air mixture and exhaust charge into its cylinders.
U.S. Pat. No. 3,744,463 to James McFarland, Jr. describes an improved independent runner intake manifold which has a step or sudden enlargement proximate the exit of each runner into an associated inlet port of an engine. The step is located where mixture stream velocity is relatively low in comparison with mixture stream velocity elsewhere in the same velocity profile. Mixture stream velocity profile simply means the curve across a cross section of a runner defined by the total of the velocity magnitude vectors at the large cross section. The patent observes that this sudden enlargement acts as a dam or block in preventing reverse flow of exhause gases up a runner towards a carburetor. The patent states that the sudden enlargement or step can dissipate the energy of a pressure pulse traveling up a runner as well as directing it back towards the inlet port.
Today internal combustion engine design is very sensitive to the amount of exhaust gas emissions of the pollutants of unburned hydrocarbons, carbon monoxide and the oxides of nitrogen. In addition, modern engine design is becoming more sensitive to problems of fuel economy. In recent years, the problem of fuel economy has been aggravated because of emission control standards which have had a tendency to reduce economy.